Electric power steering apparatuses are conventionally known, which comprises a power steering unit driven by an electric motor, and a control unit including a microcomputer and a motor drive circuit. In operation, a steering torque generated when the steering wheel is turned and a vehicle velocity are detected and, based on signals corresponding to the detected steering torque and vehicle velocity, the control unit performs PWM (pulse-width modulation)-drive control of the electric motor to generate a steering assist power to thereby reduce the necessary steering power to be applied by the driver.
The motor drive circuit includes an FET (field-effect transistor) bridge circuit and a predrive circuit for applying a drive signal voltage to gates of FETs of the FET bridge circuit. The motor drive circuit performs drive control of the electric motor in accordance with a PWM drive signal supplied from the microcomputer.
In the FET bridge circuit, drains of those FETs at a positive potential are connected to a power supply or source, sources of those FETs at a negative potential are grounded, and sources of the FETs at the positive potential and drains of the FETs at the negative potential are connected together to form output terminals to which the electric motor is connected. The FETs at the positive potential are driven via a voltage step-up circuit (generally called as “booster converter” or “step-up converter”) so that a smooth steering feel can be obtained. One example of such known electric power steering apparatus is disclosed in Japanese Patent No. 2,864,474.
FIG. 9 shows a circuit diagram of a motor controller incorporated in another conventional electric power steering apparatus. The motor controller 100 includes an interface circuit 123 having an analog to digital (A/D) converter that converts analog signals (including a steering torque signal from a steering torque detecting section 106, a vehicle velocity signal from a vehicle velocity detecting section 107 and an engine speed (r.p.m.) signal from an engine speed detecting section 124) into digital signals. The digital signals are supplied from the interface circuit 123 to a microcomputer 122.
Another interface circuit 125 converts drive currents detected by motor current sensors 118, 119 into digital signals and delivers the digital signals to the microcomputer 122. Still another interface circuit 126 delivers an exciting current from an RD (resolver digital) converter 127 to a resolver 102 and also delivers an output signal from the resolver 102 to the RD converter 127. The RD converter 127 generates, on the basis of the output signal from the resolver 102, a rotational angle signal indicative of the rotational position of a rotor of an electric motor (three phase brushless motor) 101 and supplies the rotational angle signal to the microcomputer 122. A motor drive circuit 116 is composed of a predrive section 140 and an inverter circuit 150 having six power FETs. The predrive section 140 includes a predrive circuit 128 for supplying a drive signal to FETs at a positive potential of the inverter circuit 150, and a voltage step circuit 130 that steps up a line voltage of a power supply battery) 129, for example, from 12-volts to 24-volts before the line voltage is supplied to the predrive circuit 128.
The microcomputer 122 is connected with a crystal oscillator 131 and capacitors 132, 133 that are provided externally of the microcomputer 122 so that in the microcomputer 122 an oscillating frequency of the crystal oscillator 131 is divided to generate a frequency of PWM signal (PWM frequency) that is used to for driving the brushless motor 101.
The crystal oscillator 131 and the capacitors 132, 133 are also connected to the RD converter 127 so that in the RD converter 127, the oscillating frequency of the crystal oscillator 131 is divided to generate a frequency of an exciting signal (exiting frequency) that is used for driving the resolver 102.
With the motor controller 100 thus constructed, since the FETs at a positive potential of the inverter circuit 150 are driven by the motor drive circuit 118 via the voltage step-up circuit 130 of the predrive section 140, a smooth steering tough or feel can be obtained.
However, when another in-vehicle device or unit is in motion or when the battery 129 undergoes degradation, driving of the motor 101 of the power steering apparatus will involve an additional quantity of current drawn from the battery 129, causing the line voltage of the motor vehicle to drop abruptly. In this instance, due to such abrupt drop in the line voltage (i.e., the voltage level of the battery 129 to the motor controller 100), the voltage appearing after the voltage step-up circuit 130 also dips below the normal. As a consequence, an FET gate drive signal delivered from the predrive circuit 128 involves a voltage drop, which increases ON-resistance of the FETs, tending to fluctuate the motor drive voltage. This causes the motor output to fluctuate, resulting in deterioration of the steering wheel Furthermore, since the FM with increased ON-resistance will generate heat upon conduction, the ON-resistance becomes higher, making it more difficult to obtain a smooth steering feel.
The foregoing problems will be discussed in further detail with reference to FIGS. 10–13. FIG. 10 is a graph showing the relationship between the terminal voltage of the battery (power source) and the discharge current of the battery. In the graph, a curve designated by A1 represents a terminal voltage versus discharge current characteristic curve obtained when a battery is in the initial state and hence degradation of the battery does not occur, while a curve designated by A2 represents a terminal voltage versus discharge current characteristic curve obtained when a battery is in a degraded condition.
As appears clear from the curve A1, the terminal voltages decreases as the discharge current increases. From this, it will be readily understood that when another in-vehicle device or unit is driven while the electric power steering device is operating, the discharge current increases and the terminal voltage decreases conversely. In the case where a 12V battery is used as a power source, the terminal voltage normally varies in a range of from 10V to 12V. However, when an abrupt voltage drop occurs due to driving of another in-vehicle device during operation of the electric power steering apparatus, the terminal voltage may drop to 8V or so. As is evident from the curve A2, when the battery undergoes degradation, the terminal voltage drops steeper than that of the non-degraded battery (e., the battery in the initial state represented by the curve A1) as the discharge curt increases.
FIG. 11 shows the structure of an FET used in an FET bridge circuit such as the inverter circuit 150 shown in FIG. 9. As shown in this figure, the FET has a source terminal S, a drain terminal D and a gate terminal G. In FIG. 11, the drain-source voltage, gate-source voltage and drain current are denoted by VDS, VGS and ID, respectively. The gate-source voltage will be hereinafter referred to as “gate voltage”.
FIG. 12 is a graph showing current versus voltage characteristic curves of the FET shown in FIG. 11, which are plotted under the condition that Td=25° C. where TJ is the junction temperature. In the graph, the horizontal axis represents the drain source voltage VDS and the vertical axis represents the drain current ID. Furthermore, curves designated by B1, B2, B3, B4, B5 and B6 represent current versus voltage characteristics obtained when the gate voltage VGS is 10V, 9V, 8V, 7V, 6V and 5V, respectively. As is apparent from FIG. 12, gradients of the respective current versus voltage characteristic curves B1, B2, B3, B4, B5 and B6 become small as the gate voltage VGS decreases. That is, ON-resistance (internal resistance) of the FET increases with a decrease in gate voltage VGS, and the gate voltage VGS decreases with the drain current ID. Furthermore, for the gate voltage VGS of 5V, the drain current ID does not reach a value of 80A, which is necessary for driving the motor of the electric power steering apparatus, even when the source-drain voltage VSD is increased.
It appears clear from FIG. 12 that in order to obtain the necessary motor-driving drain current (ID=80A), the gate voltage VGS must be 6V or higher. For the gate voltage VGS=10V, the necessary motor-driving drain current (ID=80A) can be obtained when the drain-source voltage VDS is 0.6V. Similarly, for the gate voltage VGS=6V, the necessary motor-driving drain current (ID=80A) is obtained when VDS is 1.5V. This means that the motor drive voltage decreases with the result that the steering feel is deteriorated To deal with this problem, it has been a general practice that a voltage Step-up circuit increases the line voltage so that the FETs are driven by a gate drive signal with sufficiently high voltage value. As previously discussed, the conventional motor controller having such voltage step-up circuit still encounters a problem when another in-vehicle device or apparatus is driven while the electric power steering apparatus is in motion.
FIG. 13 is a graph showing the temperature-dependent characteristic of the FET ON-resistance observed under the condition that VGS=6V and ID=37.5A. As is apparent from FIG. 13, the ON-resistance of FETs increases with an increase in temperature.
It will be appreciated from FIGS. 10–13 that as the gate voltage VGS decreases, the ON-resistance of the FETs increases. An attempt to control the current at a constant value will increase Joule heat produced in each FET (as represented by Q=I2Rt). The temperature of the FET is thus increased, and the ON-resistance of the FET increases as previously discussed with reference to FIG. 13. With an increase in ON-resistance, the current flowing in the FET decreases with the result that the motor current is reduced In this instance, since feedback control is performed to keep the motor current constant, the duty ratio is increased to make the motor current equal a target current. This process is, however, accompanied by further generation of Joule heat and a further increase of ON-resistance. Consequently, the motor output is caused to slightly vibrate or fluctuate about a given value with the result that the steering feel is deteriorated.